A superabsorbent polymer (SAP) is a type of synthetic polymeric materials capable of absorbing moisture from about 500 to 1000 times its own weight, and also called SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields or the like.
As a preparation process for such superabsorbent polymers, a process by a reverse phase suspension polymerization and a process by a solution polymerization have been known. Of them, preparation of the superabsorbent polymer by reverse phase suspension polymerization is disclosed in, for example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, S57-198714, etc. Further, preparation of the superabsorbent polymer by the solution polymerization further includes a thermal polymerization method in which a water-containing gel polymer is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays onto a belt to be polymerized and dried at the same time.
Important performances of hygiene products such as diapers, etc. are intake time and wet back. As diapers become thinner, permeability is also emerging as an important performance for efficient diffusion of a material absorbed into the superabsorbent polymer used in diapers. For better dryness of diapers, it is important to improve suction power of sucking urine from pulps.
To improve gel bed permeability, a method of increasing the degree of internal crosslinking or gel strength of the superabsorbent polymer or a method of coating the surface thereof with silica or inorganic particles is known. On the contrary, to improve suction power, the degree of internal crosslinking of the superabsorbent polymer must be decreased and the content of silica or inorganic particles on the surface thereof must be decreased.
In the prior art, however, there have been problems that increase of the silica coating for improvement of gel bed permeability of the superabsorbent polymer decreases absorbency under load and suction power. Accordingly, there is an urgent demand for a preparation method capable of improving gel bed permeability, absorbency under load, and suction power at the same time.